Nucleation of Dynamic Recrystallization at the Grain Boundaries of Copper Bicrystals

Abstract

Orientation-controlled copper bicrystals containing symmetrical 70o [0 0 1] tilt boundaries were
deformed in tension at 923 K and at three initial strain rates from 4.2 x 10-5s-1 to 4.2 x 10-3s-1. The load was
applied parallel to the grain boundary so as to eliminate grain boundary sliding. The nucleation of dynamic
recrystallization (DRX) was investigated using optical microscopy and orientation imaging microscopy
methods. After grain-boundary migration (GBM) and bulging, nuclei appeared behind the most deeply bulged
grain boundary regions. The critical strain for nucleation was less than one-half of the peak strain and largely
independent of the strain rate. At a fixed strain, nucleation is more frequent and the grain size finer as the strain
rate is increased. All the nuclei were twin-related (Σ3) to the matrices. Furthermore, most of the twinning plane
traces were parallel to the inactive slip traces of the bicrystals. This indicates that twin variant selection is
essentially unaffected by dislocation motion. The observed mechanism of nucleation of DRX is discussed in
relation to the occurrence of GBM and twinning.